Well pressure data testing method



Nov. 12, 1968 v. R. SLOVER, JR, ETAL 3,410,137

WELL PRESSURE DATA TESTING METHOD Filed June 6, 1966 2 i n 3O z H, v V

zz; H 35 y a m l I4 'iiL lilllilmllir VASEL R. SLOVER,JR.

""' EDWIN E. GLENN,JR.

MALCOLM K. STRUBHAR INVENTORS ATTORNEY United States Patent 3,410,137 WELL PRESSURE DATA TESTING METHOD Vasel R. Slover, Jr., Irving, Edwin E. Glenn, Jr., Dallas, and Malcolm K. Strubhar, Irving, Tex., assignors to Mobil Oil Corporation, a corporation of New York Filed June 6, 1966, Ser. No. 555,465 18 Claims. (Cl. 73-155) ABSTRACT OF THE DISCLOSURE A method for determining th bottomhole pressure for a well in which the production tubing is closed to the inflow of liquid through the inlet and gas is injected into the annulus between the tubing and casing under sufficient pressure to restrict the rise of liquid. The level of the liquid and the pressure of the gas are then monitored.

oil wells periodically are placed on test in order to gain information on the pressur within such wells under various conditions. As is well known in the art, the rate of production of a well varies with the differential between the pressure within the formation penetrated by the well and the pressure within the well opposite the formation. This latter pressure is commonly termed the bottomhole pressure of the well. The bottomhole pressure of a well at any given production rate is designated as the operating pressure of the well for this production rate. If the formation pressure and the operating pressure for a given production rate are known, it is possible of course to calculate the pressure differential necessary to maintain this rate.

The pressure of a formation in the portion thereof adjacent a well may be determined by the so-called pres sure build-up test in which the well is shut in for such time as is necessary to establish a pressure equilibrium between the well and the formation. During this shut-in period the bottomhole pressure of the well is measured so that a pressure build-up curve, i.e., a plot of pressure versus time, may be obtained. The pressure measurements may be obtained by means of a pressure measuring and recording instrument, commonly termed a pressure bomb, which is positioned at a suitable location within the well. The operating pressure of the well may likewise be determined with such pressure recording devices while the well is on production. Thus, while the well is produced at a certain rate, a pressure bomb may be positioned within the weIl adjacent the formation and the desired measurements taken. The well during such test may be produced at two or more different rates in order to obtain a correlation of operating pressure and production rate. A further correlation between operating pressure and production rate may be obtained from a drawdown test. In this procedure, which may be carried out following a pressure build-up test, the well is opened to production during which time the bottomhole pressure and production rate are measured.

Bottomhole pressure measurements of a well taken either as a function of production rate or time or both give valuable information regarding various factors such as the permeability of the formation and possible damage to the face of the formation at the wellbore. Numerous analytical techniques ar known for determining various well and/ or formation characteristics on the basis of such lCc pressure-rate-time data. For example, in the paper by D. R. Horner, Pressure Build-Up In Wells, Proceedings Third World Petroleum Congress, Section II, Drilling and Production, The Hague, 1951, page 503, there is disclosed an analytical technique for determining formation conductivity and reservoir pressure.

As noted previously, bottomhole pressure measurements may be obtained during well tests through the use of suitable pressure bombs. While some expense and inconvenience are involved in running a pressure bomb into a well, this usually can be accomplished in a flowing Well with not too much difliculty by simply lowering the device into the well through the production tubing on a wire line or other suitable cable. However, in some cases such lowering of a pressure recording device to the proper position in a well can be achieved only with great difliculty and expense. This is particularly true for wells which are equipped with mechanical equipment for lifting oil to the surface, i.e., pumping wells. For example, in the case of a well operated by a sucker-rod pumping unit, the rods must be removed before the tool can be lowered into the well through the production tubing. This is expensive and time consuming and in addition the well cannot be pumped as desired during the pressure testing procedure.

Numerous techniqeus have evolved from attempts to expeditiously and reliably gather pressure-rate-time data for pumping wells. Parallel strings of tubing have been run in a well so that a pressure bomb can be lowered through one string while the other string is reserved for pumping the well as desired. Oftentimes, however, the size of a Well will not premit introduction of two parallel strings of tubing. In addition, added expenses are involved in the use of an extra tubing string. Another known technique involves running the pressure bomb in the an nular space between the tubing and casing of a well. Here again, space requirements often are not fulfilled and there also is a serious danger of getting the pressure recording tool stuck within the annulus. This may necessitate the use of expensive fishing techniques in order to avoid loss of the tool and possibly involve long periods of well shut in.

In view of the difliculties involved in the use of pressure bombs, the so-called fill-up technique has been proposed as a means of obtaining pressure build-up data for pumping wells. This technique involves stopping the pump and allowing oil to accumulate in the well until the equilibrium head necessary to offset the reservoir pressure is reached. The level of the liquid within the well is noted at suitable intervals of time as it rises in the annular space surrounding the production tubing. The hydrostatic pressure gradient of the liquid column within the well then is estimated in order to calculate bottomhole pressure values.

Subsequent to obtaining a pressure build-up curve by the fill-up technique, a drawdown test may be carried out. Thus, after the well is shut in for the desired period of time to obtain a build-up curve, the well again is placed on production by starting the pump. As the pump reduces the liquid level in the annulus of the well, this level may be monitored in order to obtain an estimation of the bottomhole pressure as the well is pumping or drawing down.

The above-described fill-up and drawdown techniques enable a well to be tested without the hazards and expenses involved in lowering a pressure recording tool into from a sample of the oil Obtained at the wellhead. However, this offers only an approximation of the gravity of the oil as it exists within the well. In this regard, the oil as it enters the well from the formation may have entrained and dissolved therein an unknown and substantial amount of gas which will reduce the specific gravity thereof by an unknown amount. Also, the well may produce'varying amounts of water which, although it affects the hydraulic gradient of the liquids within the well to a lesser extent than gas, also may introduce an error.

Another disadvantage in the fill-up test is that although the well nominally is shut in during the test, the well still is producing relative to the formation in the sense that oil is flowing from the formation into the well. Thus, a truly shut-in condition of the well with respect to the formation is not achieved until the equilibrium head of fluid within the well necessary to offset the formation pressure is reached. In addition, the information obtained during drawdown testing of the well is not truly representative of conditions existing when the production rate of the well is substantially equal to the rate at which fluids flow from the formation into the well. In this regard, as the pump is started up again, a portion of the liquid produced from the well through the production tubing is liquid which'has accumulated in the well during the shut-in period. While the well of course produces fluid from the formation as the bottomhole pressure is lowered, i.e., as the hydrostatic head in the well is decreased, transient conditions are set up during this period which make subsequent analytical evaluation of the test complicated and subject to error.

In accordance with the present invention, there is provided a new and improved well testing method which is alleviative of the limitations and disadvantages noted above and which is particularly well adapted for use in wells which are produced by artificial lifting techniques.

The instant invention is carried out in a well having a production tubing and an open production interval within the well through which fluids flow from the formation and accumulate in the well in a space exteriorly of the production tubing. This exterior space may be the annulus defined by the outer and inner perimeters of the production tubing and casing, respectively, or in those instances where the well is uncased opposite the production formation, this exterior space may be the annulus defined by the outer perimeter of the production tubing and the face of the formation. The tubing is provided with an inlet through which the accumulated fluids pass to the interior of the tubing and thence to the surface of the well.

. In carrying out the invention, the production tubing is closed to the inflow of liquid through the inlet thereof and gas is injected into the aforementioned exterior space, e.g., the annulus between the tubing and casing, under sufiicient pressure to restrict the rise of the liquid within the exterior space from the level of the liquid at the time of closing the production tubing. Preferably, the gas is injected at a rate sufficient to maintain the liquid level within the exterior space substantially constant. While the gasis injected as described above, the level of the liquid within the exterior space and the pressure of the gas within the exterior space above the liquid are measured continuously or at suitable time intervals. From these measurements the bottomhole pressure for the well over the period of the test may be determined.

In accordance with a preferred embodiment of the present invention, the well is produced for a. specified time interval and at a substantially constant rate prior to closing the production tubing as described above. While the well is so produced, the liquid level within the exterior space is measured and the production of the well is continued until a substantially constant liquid level is achieved. Thereafter, the production tubing is closed as described above and the aforementioned procedure is carried out.

In accordance with yet another aspect of the instant invention, the above procedure can be substantially reversed subsequent to the well being shut in for a suitable period in order to carry out a new and improved variable rate drawndown test. In this case, the pressure within the exterior space is gradually decreased by venting the previously injected gas therefrom. During the time the pressure is decreased, the liquid level in the exterior space and the gas pressure are monitored in order to obtain a representation of the bottomhole pressure of the well during this period.

For a better understanding of the present invention, reference may be had to the following detailed description taken in conjunction with the accompanying drawing which is a schematic illustration partly in section showing a well and attendant equipment for carrying out the instant invention.

With reference to the drawing, there is shown therein a wcllbore 10 which extends from the earths surface 11 and penetrates a subterranean oil producing formation 12. Thewellbore may be provided with a casing string 14 which is set to the top of the producing formation 12 and cemented as indicated by reference numeral 16. The uncased portion of the well indicated by bracket 18 defines an open production interval within the well through which fluids may enter the well from the surrounding subterranean formation 12. Although, in the well illustrated, the open production interval is defined by an uncased section of the well, it will be understood that other suitable completion arrangements may be used. For example, rather than the open hole completion shown, the well may be cased and cemented to its total depth and the casing and surrounding cement sheath then gunor jetperforated opposite the formation 12 in order to define an open production interval for the well. Such procedures of providing an opening in a well to the flow of subterranean fluids are well know in the art and therefore will not be described further.

A production tubing string 20 is disposed within the well and extends from the surface of the well or wellhead 21 through a packer 22 to a suitable depth within the well, for example, adjacent the formation as shown. Liquid from the subterranean formation 12 accumulates in the annulus 23 of the well and is produced through the interior of the tubing string 20 by means of a suitable bottomhole pumping system, described in detail hereinafter,

, and carried to the surface of the well where it passes into a suitable gathering line 24. Gathering line 24 is provided with suitable flow rate metering and recording means 26.

The well also is provided with a second tubing string 28 which extends from the wellhead 21 through the packer 22 and thus provides for fluid communication between the surface of the well and the annulus 23 surrounding tubing 20. A suitable input line 30 is connected to tubing 28 and extends to a source 32 of gas under pressure. Input line 30 is provided with valve means 34 for regulating the flow of gas from source 32 into the annulus 23 surrounding tubing string 20. A second valve means 35 also is provided for venting the annulus 23 to the atmosphere or other suitable low pressure zone. The input line 30 also is provided with suitable liquid detecting means 36 for measuring and recording the level of liquid in the annulus 23 and pressure responsive means 38 for measuring and recording the gas pressure above the liquid column. Liquid level detecting means 36 may be of any suitable type. In most cases, however, it will be advantageous to utilize a liquid-level detector of the sonic type such as that at the top of tubing string 20 and is connected by means of asuitable cable arrangement 46 to a walking beam 48 which is driven by an adjustable speed prime mover 50. Prime mover 50 is connected to the walking beam 48 for actuation thereof by means of a pitman 52. The lower end portion of the production tubing 20 is provided with a perforated tail-pipe section 20a which serves as an inlet for the flow of liquid accumulated within the well into the tubing string 20. Other inlet means may of course be provided. For example, the tubing 20 may simply terminate with an open end immediately below the standing valve. In most cases, however, a perforated inlet section such as that shown is desirable, particularlywhere sand is entrained in the formation fluids.

The operation of the pumping system illustrated is well known in the art and accordingly will be described only briefly. As shown in the drawing, the plunger assembly 42 is at the bottom of the downstroke and the traveling and standing valves are seated in their respective valve seats. As the upstroke of the pump is initiated, the plunger assembly 42 moves upwardly and liquid accumulated in the annulus 23 exteriorly of the production tubing 20 flows through the standing valve up into the working barrel section of the tubing between the standing and traveling valves. As the plunger completes its upstroke, the standing valve drops back upon its valve seat and prevents the liquid which has entered the working barrel portion of the tubing from flowing back into the well. On the succeeding downstroke of the plunger assembly 42, the liquid within the working barrel is displaced upwardly through the traveling valve into the section of the tubing above the plunger assembly. On the next succeeding upstroke of the pump, the liquid thus displaced past the traveling valve is lifted upwardly within the tubing and, assuming that the tubing string is filled, a slug of liquid is forced into the gathering line 24. From the foregoing description, it will be recognized that upon stopping the action of the pump the standing and working valves will remain seated, thus closing the tubing 20 to the inflow of liquid through inlet 20a.

It is to be recognized that the well structure and equipment thus far described are merely exemplary and that other suitable systems may be utilized in carrying out the invention. For example, in a well which is not equipped with a packer such as that shown at 22, the second tubing string 28 may be dispensed with and gas inlet line 30 may be directly connected to the casing 14. This arrangement, of course, is functionally equivalent to that shown in the drawing since in either case means are provided for injecting gas into the annulus 23 or other space exteriorly of the tubing 20.

In making a pressure build-up test in a Well in accordance with the instant invention, it is preferred to establish first the operating bottomhole pressure of the well for a given pumping rate. This is accomplished in the preferred embodiment of the invention by producing the well at a constant rate and monitoring the liquid level in the annulus 23. The well is produced at a rate less than that required to cause pumping off, i.e., lowering of the liquid level in the annulus to a point below the inlet of the production tubing 20. As the well is pumped at this given rate, it ultimately will reach an equilibrium condition at which the fluid flow rate from the formation into the well will substantially equal the fluid flow rate from the well through the production tubing 20. At this equilibrium condition, the liquid level within the annulus 23 will be substantially constant with only a small amount of fluctuation due to the action of the pump as it intermittently withdraws fluid from the annulus into the flow tubing 20 on each upstroke of the plunger and traveling valve assembly 42. By monitoring the liquid level within the well to determine this substantially constant operating level, the back pressure due to the hydrostatic head of the liquid operating against the formation face may be determined.

During this phase of the well test, the Well may be pumped at any constant rate which will not result in the lowering of the liquid level below the top of open production interval 18 or the inlet of the tubing 20 in the event the inlet is positioned above the open production interval. Preferably, however, the flow will be pumped at a rate such that the ultimately achieved constant operating level is near the top of the open production interval 18. This will greatly reduce the possibility of error due to an unknown gravity value of the liquid within the well and also will provide for a relative maximum pressure change during the pressure build-up portion of the test. It is particularly important to maintain a relatively low operating liquid level in those instances Where the well produces at a relatively high gas-oil ratio since the condition is most conducive of error due to unknown hydraulic gradients existing within the Well. In most cases, particularly where the well is known to produce at a gas-oil ratio of on the order of 200 standard cubic feet per barrel or more, the operating liquid level within the annulus should be maintained at a distance not greater than 500 feet above the top of the open production interval. Preferably, the operating liquid level will be maintained at a distance not greater than 50 feet above the top of the open production interval.

The constant production rate of the well should be maintained for an adequate period in order to insure that a stable equilibrium condition is reached and to provide adequate data for subsequent analysis of the test. The well should be pumped at a constant rate for a period of at least six hours. Preferably, the Well is pumped at a constant rate for a period of at least 24 hours.

After the Well is pumped at the desired constant rate and the substantially constant liquid level in the annulus 23 is reached, the pump is stopped, thus closing the production tubing 20 to the inflow of liquid through the inlet 20a thereof and effectively shutting in the well. At this time, gas from source 32 is injected through line 30 and tubing string 28 into the annulus 23 in order to restrict, i.e., prevent or at least retard, the rise of the liquid level therein from the operating level existing when the pump is stopped. Preferably, the gas is injected at a rate suflicient to maintain the level of the liquid substantially constant and at the operating level existing at the time the well is shut in. This is accomplished by monitoring either continuously or intermittently the liquid level within the well and adjusting the gas injection rate as necessary through proper manipulation of valve 34.

During the gas injection step, the gas pressure in the annulus is measured and recorded continuously or at suitable time intervals. While the gas pressure at the surface of the liquid in the annulus 23 is the parameter of interest, it will be recognized that in most instances this can be determined with a reasonable degree of accuracy by measuring the gas pressure in inlet line 30 at some point downstream of valve 34. Even in relatively deep Wells where a significant pneumatic gradient exists Within the annulus, the gas pressure on the column of liquid can be readily calculated from measurements obtained at the surface by giving due consideration to the weight of the gas column above the liquid. Accordingly, the step of measuring the pressure within the exterior space of the tubing above the liquid level is intended to cover measurements taken at the surface such as illustrated in the drawing as well as measurements taken directly at the liquid level. In either case, of course, a representation of the gas pressure immediately above the column of liquid is obtained such that this pressure is determined either directly or through suitable calculations which are well known to those skilled in the art.

It will be recognized that from the data obtained from the above-described gas pressure and liquid-level measurements, a pressure build-up curve for the well may be plotted. This curve is somewhat analogous to those pressure build-up curves obtained by the conventional fillup test. However, assuming the liquid level is maintained substantially constant, the pressure build-up curve obtained by the instant invention will be truly representative of a condition in which the well is shut in relative to the formation, i.e., a condition under which fluid does not flow from the formation 12 into the annulus 23 of the well.

As noted above, the liquid level during the gas injection step preferably is maintained substantially constant with only minor variations, e.g., on the order of a few feet. However, it will be recognized that as a practical matter difiiculty may be experienced in injecting ga in response to the monitored liquid level in the annulus 23 with an exactness of rate sufiicient to accomplish this. In most cases, however, no difliculty should be experienced in maintaining the liquid column in annulus 23 at a level within :30 feet of the original operating level, and it is preferred in carrying out the instant invention to maintain the liquid level within at least this range. Preferably, this liquid level is maintained within a range of feet of the original operating level.

If difiiculty is experienced in maintaining the liquid level in the annulus 23 within a reasonably narrow range, it is preferred to operate such that any error is on the side of a low gas injection rate in order to avoid forcing an appreciable quantity of liquid from the well back into the formation 12. Where, as is preferred, the original operating level of the liquid in annulus 23 is relatively low, particular care should be exercised in order to avoid depressing the liquid level below the top of the open production interval 18 with the attendant possibility of injected gas entering the formation 12. In those instances in which the liquid level rise is merely retarded somewhat by the injection of gas rather than substantially prevented, there will be some flow of formation fluids into the well. It will be recognized, however, that this flow will be much less than in those cases in which the conventional fill-up technique is practiced.

In some wells, the influx rate from the formation into the wellbore may be quite high. In this case, it may be difficult to inject gas at a rate sufiicient to maintain the liquid level in the annulus substantially constant during the period immediately following the closing of the production tubing. Under such circumstances, it may be desirable to initiate gas injection into the annulus shortly before closing the production tubing.

The gas injection step is carried out over a time interval necessary to obtain the desired pressure build-up curve. In some instances, it will be desired to obtain a complete pressure build-up curve. In such a case, gas injection should be continued until a stable equilibrium pressure is achieved within the annulus above the liquid column. That is, the gas will be injected until the gas pressure in annulus 23 is such that additional injection of gas with the resultant increase in pressure causes a progressive reduction of the liquid level. At this point the bottomhole pressure, which may be calculated from the known head of liquid and the gas pressure in the annulus, is equal to the formation pressure. In some instances, it will be unnecessary to carry out the gas injection step to this conclusion since the final build-up pressure can be determined accurately by extrapolation of the results of a partial build-up test. In any case, where the well is initially produced at a constant rate as described above, gas injection should be carried out for a period of at least one-tenth of the time during which the well was pumped at this constant rate. Preferably, the gas injection step is carried out for a period of at least onefourth of this time interval. Thus, if as is preferred, the well is maintained on a constant production rate for a period of 24 hours, gas injection into the annulus preferably is continued for a period of at least six hours subsequent to stopping the pump and closing the tubing to the inflow of liquid through the inlet thereof.

While production of the well at a constant rate prior to closing the tubing string 20 is preferred, valuable information may be obtained merely by carrying out a pressure build-up test in accordance with the instant invention without first maintaining a constant production rate. In this case, however, it usually will be desirable to at least monitor the production rate (even though it is not constant) and also monitor the liquid level in the well prior to shutting in the tubing. These measurements should be taken over a period of at least six hours in order to establish sufficient data for a variable rate analy- SIS.

In some instances, of course, time limitations may preclude measuring the production rate and liquid level for any significant interval prior to shuttingin the tubing. In this case, if the instantaneous rate, can be measured, and this rate may be presumed to have prevailed for some period of time, a pressure build-up test carried out in accordance with the instant invention may provide valuable information. Of course, in this instance, the liquid level should be monitored or should be determined at the time the tubing string 20 is closed.

In accordance with the instant invention, a variable rate drawdown test may be run after carrying out a pressure build-up test. In carrying out this aspect of the invention, after the'gas pressure in the annulus is stabilized or before such stabilization if a complete build-up test is not run, the gas is vented from the annulus by opening the valve 35 in line 30, the valve 34 normally being closed at this time. Preferably, the pressure build-up test is carried substantially to completion before initiating the drawdown test. As the pressure in the annulus is gradually reduced, the liquid level rises as liquid fiows from the formation 12 into the well under the progressively decreasing bottomhole pressure and the attendant increasing pressure differential between the formation and well. In order to provide adequate and accurate information for subsequent analysis, the rate of gas pressure decrease during the drawdown test preferably is no greater than the rate at which gas pressure was increased during the build-up test. Thus, the gas should be vented from the annulus at a rate no greater than that at which it was initially injected.

During drawdown of the Well, the liquid level in the annulgs 23 and the gas pressure above the column of liquid are measured and recorded as before. From this data, and assuming the cross-sectional area of the exterior space occupied by the liquid is known, appropriate correlations can be made between the bottomhole pressure and the production rate of the well relative to the formation, Le, the rate at which liquid flows from the formation into the annulus. Since the tubing 20 is maintained in the closed condition during this period, the measured rise of the liquid level in the annulus 23 is an accurate indication of flow rate into the well.

Having described certain specific embodiments of the instant invention, it will be understood that further modifications thereof may be suggested to those skilled in the art, and it is intended to cover all such modifications as fall within the scope of the appended claims.

What is claimed is:

1. In a method of testing a well penetrating a subterranean formation and adapted for the recovery of liquids therefrom, said well havinga production tubing with an inlet within said well and an open production interval through which liquids flow from said formation and accumlate in said well in a space exteriorly of said production tubing, the steps comprising:

(a) closing said production tubing liquid through said inlet;

(b) injecting gas into the space within said well exteriorly of said tubing under suflicient pressure to restrict the rise of the liquid within said exterior space from the level of said liquid at the time of closing said production tubing; and

(c) while injecting gas in accordance with step (b) measuring at least intermittently the level of the liquid within said exterior space and the pressure of the gas within said space above said liquid level.

2. The method of claim 1 wherein said gas is injected to the inflow of at a rate sufficient to maintain said liquid level within :30 feet of the level existing at the time of closure of said production tubing.

3. The method of claim 1 wherein said gas is injected at a rate suflicient to maintain said liquid level within 10 feet of the level existing at the time of closure of said production tubing.

4. The method of claim 1 wherein said gas is injected at a rate sufiicient to maintain said liquid level substantially constant.

5. The method of claim 1 further comprising the steps of decreasing the gas pressure in said exterior space by venting gas therefrom and while decreasing said pressure measuring at least intermittently said pressure and the level of the liquid within said space.

6. The method of claim 5 wherein gas is vented from said exterior space at a rate no greater than the rate at which gas was injected in step (b) of claim 1.

7. The method of claim 1 further comprising, prior to step (a), the steps of producing liquid from said well through said production tubing and, while producing said well, measuring at elast intermittently the flow rate from said tubing and the liquid level in said exterior space.

8. The method of claim 1 further comprising, prior to step (a), the steps of measuring at least intermittently the liquid level in said exterior space and producing liquid at a substantially constant rate from said well through said production tubing until a substantially constant operating level of liquid in said exterior space is achieved.

9. The method of claim 8 wherein said operating level is above the top of said open production interval by a distance of not more than 500 feet.

10. The method of claim 8 wherein said operating level is above the top of said open production interval by a distance of not more than 50 feet.

11. The method of claim 8 wherein said substantially constant producing rate is maintained for a time period of at least six hours.

12. The method of claim 8 wherein said substantially constant producing rate is maintained for a time period of at least 24 hours.

13. The method of claim 8 wherein gas in injected into said exterior space for a period of at least one-tenth of the period during which said well is produced at said substantially constant rate.

14. The method of claim 8 wherein gas is injected into said exterior space for a period of at least one-fourth of the period during which said well is produced at said substantially constant rate.

15. The method of claim 8 wherein said gas is injected at a rate suflicient to maintain said liquid level within :10 feet of the level existing at the time of closure of said production tubing.

16. The method of claim 8 further comprising the steps of decreasing the gas pressure in said exterior space by venting gas therefrom and while decreasing said pressure measuring at least intermittently said pressure and the level of the liquid within said space. v

17. The method of claim 8 wherein gas is vented from said exterior space at a rate no greater than the rate at which gas was injected in step (b) of claim 16.

18. In the testing of a well penetrating a subterranean formation and adapted for the recovery of liquids therefrom, said well having a production tubing with an inlet within said well, an open production interval through which liquid flows from said formation and accumulates in said well in a space exteriorly of said production tubing, and a bottomhole pumping system in said tubing for withdrawing liquid from said exterior space through said inlet and displacing said liquid through said tubing to the surface of said well, the method comprising:

(a) measuring at least intermittently the level of the liquid in said exterior space and actuating said pumping system to pump liquid from said well through said production tubing at a substantially constant rate;

(b) continuing step (a) until a substantially constant operating level of liquid in said exterior space is achieved and thereafter terminating the actuation of said pumping system to close said production tubing to the inflow of liquid through said inlet;

((2) injecting gas into said exterior space at a rate sufficient to maintain the liquid within said exterior space at a level Within :10 feet of said operating level existing at the time of closing said production tubing; and

(d) while injecting gas in accordance with step (c) measuring at least intermittently the level of the liquid within said exterior space and the pressure of the gas within said space above said liquid level.

References Cited UNITED STATES PATENTS Re. 21,383 3/1940 Walker 73-155 2,320,492 6/1943 Walker 73155 2,792,709 5/1957 Bell et al 73-155 2,818,728 1/1958 Hartline et a1. 73-155 RICHARD C. QUEISSER, Primary Examiner. JERRY W. MYRACLE, Assistant Examiner. 

